1. Field of the Invention
The present invention is directed to electro-optical display devices which include single and multi-color display devices which utilize electro-optical light valves such as a liquid crystal cell for controlling transmittance of a passive light from the display device.
2. Prior Art
Many different types of passive display devices are known. Examples of known passive display devices include electrophoretic, electrochromic, liquid crystals and ferroelectric displays.
Liquid crystal displays can be based on dynamic scattering as discussed in an article by G. H. Heilmeier, L. A. Zanoni, and L. A. Barton, Proceedings of the IEEE, Vol. 56, No. 7, July, 1968, pp. 1162. In addition, liquid crystal displays may be used as rotational cell as discussed in German Offenlegungsschrift Nos. 21 58 563 and 22 02 555.
Ferroelectric displays may use linear or quadratic electro-optical refraction effects in a ferroelectric ceramic as discussed in articles by G. H. Haertling and C. E. Land, Journal of American Ceramic Society, Vol. 54, 1971, page 1 and by J. R. Maldonado and A. H. Meitzler, Proceedings of IEEE, Vol. 59, 1971, page 368. In addition, ferroelectric displays may utilize electro-optic scattering effect in the ferroelectric ceramics as discussed in an article by W. D. Smith and C. E. Land, Applied Physics Letter, Vol. 20, 1972, page 169.
Electrochromic displays may utilize an electrochromic effect, for example, in a solid state film of WO.sub.3 or MoO.sub.3 with a redox reaction in the boundary layer with electrolytes such as H.sub.2 SO.sub.4 as discussed in the article by S. K. Deb, Appl. Opt. Suppl., Vol. 3, 1969, page 192.
Each of these types of display devices possesses essentially three fundamental advantages. These advantages are a small consumption of electrical energy, a contrast which is largely independent of the ambient brightness, and a flat type of construction. However, there are some disadvantages. When the display is operated in a transmission mode with a light source located behind the display, the advantage of a flat type of construction is lost.
In a reflecting mode of operation, the ambient light which enters from the viewers side is turned back or reflected back by a reflecting layer in the direction toward the viewer through the display where it is spatially modulated. In the known display devices, this type of operation has the disadvantage which is that a relatively good readability of the display is only possible under restricted viewing conditions. For example, the incident light may cause a shadow of the display element to be cast upon the reflecting surface which shadow may interfere with the display. In addition to the above problems, many of these displays, such as liquid crystal displays which rely on a field effect basis require polarizers, and the polarizers will weaken the representational contrast which will further reduce the viewing conditions.